Microwave slow wave dielectric structure and electron tube utilizing same



0d. 28, 1969 KARP ET AL 3,475,642

MICROWAVE SLOW WAVE DIELECTRIC STRUCTURE AND ELECTRON TUBE UTILIZINGSAME Filed Aug. 10, 1966 M/VENTORS I ARTHUR KAPP DOA/440 wwszow HHPBEQTJ. SHAW 62 BY 1..

145 5 I Izly. 5 MA I A FOR/V580" United States ABSTRACT OF THEDISCLOSURE A microwave slow wave structure is provided which comprisesan array of high Q dielectric resonator elements within a shield or awaveguide, each of which may be circular in shape. The individualresonator elements are coupled together solely by their externalmagnetic fields. The degree of coupling is varied by the spacing of thearray.

This invention relates to microwave circuits and more particularly toimproved dielectric structures used at microwave frequencies.

Microwave slow-wave structures find utility in microwave circuits asbandpass or stop filters, delay lines, transducers, oscillator frequencycontrols, and in other capacities. Slow-wave structures may be installedin waveguides by providing discontinuities at proper spacings, such asby inserting irises at repeated distances of onehalf the resonantwavelength. However, heretofore there have not been available compact,high Q resonators, sharp cut-off filters, and the like with bandpasscharacteristics which could easily be varied.

Accordingly, one object of the present invention is to provide slow-wavestructures which are economical and compact, and which can be providedwith any frequency characteristic within a broad range, more easily thancould be provided with structures available heretofore.

Another object is to provide a low loss slow-wave system composed of aplurality of individual resonator elements, wherein selectivity or passband of the system is varied by varying the spacing of the elementswithout changing the center frequency of the pass band.

Still another object of this invention is to provide a high Q resonatorsystem composed of a plurality of individual resonator elements whereinthe frequency of resonance of the system is varied by varying thespacing of the elements.

Yet another object is to provide a simple resonator which can be coupledto waveguides which propagate the TE mode.

Still another object is to provide a simple means for obtaining purityof mode propagation of waves in structures composed of dielectricresonators.

The foregoing and other objects of the present invention are basicallyrealized by a resonator structure comprising an array of high Qdielectric resonator elements, such as rutile (single crystal TiO orstrontium titanate, arranged in proximity to each other. The individualresonator elements are coupled together solely by the magnetic (H)fields external to them. The degree of coupling is varied by the spacingof the array. In what may be termed the traveling wave embodiment of theinvention the center resonant frequency of the array is the resonantfrequency of each element, and the width of the pass band variesinversely with the spacing of the elements. In what may be termed thecomposite resonator embodiment of the invention, the array of dielectricelements are suspended within a metal cylinder to form a resonator. Thefrequency of resonance, but not the band-- atent ice width can be variedby varying the spacing between the dielectric elements.

Tunnels can be provided in the elements without substantially degradingtheir resonant characteristics. Arrays with tunnels can be used to focuscharged particles, by means of the axial magnetic fields threading theelements. In another application, the dielectric elements are doped neartheir axes with chromium ions or the like, to obtain a traveling wavemaser.

The arrays serve not only to control frequency characteristics, but alsoto eliminate unwanted modes of propagation in these arrays, particularlymodes other than the TE mode. Mode purity is also achieved by providinga conductive cage about a dielectric, in an embodiment of the invention.

The novel features that are considered characteristic of this inventionare set forth with particularity in the appended claims. The inventionitself both as to its organization and method of operation, as well asadditional objects and advantages thereof, will best be understood fromthe following description when read in connection with the accompanyingdrawings, in which: I

FIGURE 1 is a perspective, partial sectional view of a resonator arrayconstructed in accordance with the invention;

FIGURE 1A illustrates one mode of coupling a waveguide to the inventionshown in FIGURE 1;

FIGURE 2 is a perspective partial sectional view of an embodiment of theinvention employed to couple electromagnetic fields to a beam ofparticles;

FIGURE 3 is a representation of the array of FIGURE 1 showing the RF.magnetic field lines present;

FIGURES 4A and 4B are perspective, partial views, of other embodimentsof the invention, used as resonators;

FIGURE 5 is a perspective view of yet another embodiment of theinvention, particularly useful in obtaining purity of mode in adielectric resonator or traveling wave structure; and

FIGURE 6 is a perspective view of still another embodiment of theinvention, which is useful for obtaining purity of mode in a dielectricresonator or traveling wave structure.

The embodiment of the invention, shown in FIGURE 1, comprises aplurality of cylindrical elements 10, 12, 14 and 16. Each element isconstructed of a high Q, high e (relative dielectric constant, at leastseveral times unity) material. When the material is an anisotropiccrystalline material, there is often a preferred orientation of thecrystalline axis with respect to the RF. electric and magnetic fields.In the case of rutile, the preferred orientation is that in which theC-axis is aligned with the axis of the cylinder. Rutile is a preferredmaterial here. The elements are mounted on supports 18 of a lowdielectricconstant material, the supports being slidably mounted in atrack 20 of a base 22, which is also constructed of a lowdielectric-constant material. While in principle metal shields are notrequired where phase velocities less than C are of interest, theelements may be disposed within a shielding container 24. The elementsare positioned at a uniform distance L from each other.

Electromagnetic waves propagating along the array of dielectric unitsshould be in the TE mode, usually important as the low-loss mode incircular guides. Magnetic field lines pass through the elements andcouple them together, as represented by the lines H in FIGURE 3. The endelements couple to the H field of the waves in input and outputwaveguides (not shown).

The substantial separation of the elements 10, 12, 14 and 16 of thearray results in the coupling being largely independent of the shape andorientation of the individual elements, e.g. the elements may berectangular instead of cylindrical, or oriented with the axis not alongthat of the array, or even each element may consist of a multiplicity ofpieces of rutile. The parameters determining the characteristics of thearray, to the first order, are the spacing L and the resonant frequencyof each element in isolation. However, thin disks, such as those withthicknesses T less than one-fourth of the diameter D', are oftenpreferable because of their circular symmetry and because their lowestresonant frequency is comfortably much lower than their next higherresonant frequency. While it is not essential to do so, it is usuallyconvenient to orient the disks with their axis of symmetry parallel tothe axis of the array, and in cases where the material is anisotropic,eg for rutile, to align the crystalline C- axis with the axis of thearray.

The average or center resonant frequency of an array has been found tobe approximately the resonant frequency of the individual elements, forall useful element spacings. However, the pass band varies inverselywith the element spacing L, so that as L decreases, the bandwidthincreases. The elements in the array of FIGURE 1 are supported to bemovable toward and away from each other to vary the bandwidth. Thecoupling between elements is effective for separations L as great astwice the diameter D of the individual elements. Slow-wave structureshave been constructed in accordance with this invention for S bandoperation, using rutile cylinders oneeighth inch in diameter andone-eighth inch thick, having resonant frequencies of 3200 megacyclesand unloaded Qs of approximately 10,000. The arrays were found tofunction as predicted, and to display positive dispersion i.e. a phaseshift, from one end of the structure to the other, which increases withfrequency, when the structure is considered as a traveling-wavestructure.

FIGURE 1A shows how the embodiment of the invention shown in FIGURE 1may be coupled to a waveguide 17, 19. Coupling is made via matchingslots in end walls of the waveguide to the end discs 10, 16 of thearray.

In another embodiment of the invention, illustrated in FIGURE 2 there isprovided an array of resonator elements 30 each having a toroidal shape.The center holes of the elements are aligned. The arrangement is used tocouple electromagnetic fields to a stream of particles passing throughthe center holes. These particles may be charged or uncharged. A cathodeapparatus 34 (or a molecular oven) generates and propagates a beam ofions or electrons, for example, through an evacuated tube 36 whichextends into the shield 32 and through the centers of the resonatorelements 30, to a target 33. The material about the hole in each elementmay be doped with ferromagnetic material or with chromium ions.

When waves of a frequency within the band propagated by the array arecoupled into and out of the array by means of the respective loops, 38,39 which are effectively within the planes of the respective first andlast elements, they set up large axial magnetic fields. These fields actupon the particles in the beam to provide focussing, bunching,deflection, population inversion, or other useful function, as is wellknown in the art.

In principle, metallic shields are not required about the dielectricarray when wave velocities in it are less than c (the free space speedof light). It may be noted that this is one of the distinctions betweenthe structures of the present invention and the heretofore knownstructure of a metallic waveguide periodically loaded with dielectricdiscs.

Thus far we have been concerned with a travelingwave structure in whichWe couple into and out of the remote ends of the array. The coupling atthe ends is matched, so that the behaviour of the structure is as if thearray were infinite. Now, let us turn to an array which is not infinitein effect, and has no couplings limited to the end elements alone. If wecouple to this finite array broadside, it can act as a single lumpedresonator. Analogous to a few turns of a helix with shorted ends or withopen free ends. In practice it is found that the fields external to asingle dielectric resonator Whether or not composite as just depicted,sometimes contain components which can lead to a deterioration of Q byreason of radiation damping, unless the resonator is shielded by anelectrically conductive enclosure.

As shown in FIGURE 4A, a group of resonator elements 40 are placedwithin and coupled to a Waveguide 42. By reason of the location withinthe waveguide no further shielding is required. The back wall 44 of thewaveguide is slidable within the guide walls forming a tuning piston.However, in some applications, e.g. where direct coupling to a coaxialcable is necessary, waveguide shielding may not be obtainable. Shieldingcan be obtained by enclosing the dielectric in a metal cylinder 46 asillustrated in FIGURE 4B. The cylinder 46 has an iris aperture 48opening into the end of a waveguide 51 through which microwaves arepropagated. Each end 52 of the cylinder is open to enable the entry ofplastic or ceramic rods for manipulating the dielectric resonators 54,and to avoid spurious resonances of the metal cavity. A coaxial cable 56extends through the side of the cylinder and has a loop 50 for couplingto the resonator. The composite resonator and shield can be used inconnection with two coaxial cables instead of a waveguide and cable, byproviding two coaxial cable loops. These are positioned near the centerof the array of elements and loosely coupled thereto. Otherconfigurations of waveguides, and/ or coaxial cables for coupling energyto and from the composite resonator will be apparent to those skilled inthe art, from the foregoing description. However, these are to beconsidered within the spirit of this invention and the scope of theclaims herein.

While the array of resonator elements described finds utility as acomposite microwave resonator, or as a pass or stop band filter, toseparate propagated microwaves according to frequency, it also findsutility in regulating the mode of its own resonance. The TE or magneticdipolar mode of a dielectric element, With the H field concentratedalong the axis of the element, is often the most important mode used inconnect-ion with paramagnetic, ferromagnetic, hall-effect, andmagnetostrictive specimens located near said axis. Other modes ofdielectric resonance often tune in at frequencies close to that at whichthe TE mode creates an effect. In many applications it is desirable toeliminate the effects of all modes other than the TE mode. Positivesuppression of unwated modes is accomplished without substantiallowering of the Q of a dielectric element by employing as the entireresonator in a filter, or as each element of an array for traveling wavepurposes, at least two of such elements 60 separated by small gaps 62 asillustrated in FIGURE 5. When thin air gaps, or films of low-loss, low-edielectric, such as Mylar films, are inserted transverse to the axis ofthe array, modes with an E component parallel to the axis 64 of thearray are detuned to a remote high frequency while the desired mode isnot appreciably affected. This is due to the fact that E fields do notreadily jump air gaps while H fields do.

Another arrangement for suppressing unwanted modes in any applicationutilizing one or more dielectric resonators is illustrated in FIGURE 6.It involves the placing of a cage 70 of thin, electrically conductivewire loops in meridional planes about the dielectric material 72, i.e.defining planes perpendicular to each other and parallel to the RF. Hfield. A mode with H field lines tending to thread a loop is damped anddetuned. The desired TE mode is not affected appreciably if the wires ofthe cage are very thin. The finite thickness of the wires somewhatdecreases Q and the resonant wave length in the desired mode but theeffect is very small where the Wire thickness is less than aboutone-fiftieth the width of the resonant element.

While particular embodiments of the invention have been describedherein, many variations and modifications will be apparent to thoseskilled in the art. For example, arrays may be constructed withferromagnetic resonator elements at their axes, or chromium dopeddielectric materials may be used at the centers of the elements toprovide traveling wave masers. Also, groups of arrays may be positionedbeside each other for mutual interaction. Therefore, the inventionshould not be considered as limited by the particular embodimentsdescribed herein, but only by a just interpretation of the followingclaims.

What is claimed is:

1. A structure for use in a microwave system comprismg:

a plurality of dielectric resonator elements, each comprising a cylinderof rutile having its crystalline C- axes aligned with its axes ofsymmetry, and the axes of symmetry of said plurality of dielectricelements being aligned with each other;

each element being resonant at a predetermined frequency; and

means for positioning said elements in an array relative to one another,with a predetermined spacing and in positions wherein radio frequencymagnetic field lines of microwaves propagated by said microwave systempass through and between said resonator elements.

2. A structure as recited in claim 1 wherein each said element is spacedfrom an adjacent element by a relatively thin dielectric material ofrelatively low loss.

3. A structure as defined in claim 2 wherein the spacing of saidelements is less than twice their diameters, whereby to assure couplingbetween the elements.

4. A structure as recited in claim 1 wherein said plurality ofdielectric resonator elements are all resonant at substantially the samefrequency; and

said structure includes means for applying microwave energy to a firstof said array of resonator elements; and

means for deriving microwave energy from a last of said array ofresonator elements.

5. A structure as recited in claim 1 wherein said plurality ofdielectric resonator elements are all resonant at substantially the samefrequency; and there is included means for applying microwave energysubstantially simultaneously to said plurality of resonator elementsfrom one side of said array.

6. A structure as defined in claim 1 wherein each of said elements has atoroidal shape with the central holes of said toroidal elements beingaligned, said structure including:

means for applying microwave energy to said elements;

and

particle generating means for directing a beam of particles through theholes in said elements;

means to couple said microwave energy to said beam of particles.

7. A structure as defined in claim 6 wherein the material surroundingthe central holes of each of said toroidal elements is doped withferromagnetic material.

8. A structure as defined in claim 6 wherein the material surroundingthe central holes of each of said toroidal elements is doped withchromium ions.

9. A structure defined in claim 1 wherein said plurality of resonatorelements includes at least three elements of disk-like shape, eachhaving a thickness no more than one-fourth its diameter.

10. A structure for use in a microwave system comprising:

a plurality of dielectric resonator elements, each constructed ofmaterial having high Q microwave frequencies, each having a relativedielectric constant at least several times unity, and each beingresonant at a predetermined frequency;

each element being enclosed in a cage of thin electrically conductivewire loops disposed in meridional planes about said dielectric material;and

means for positioning said elements in an array relative to one another,with a predetermined spacing and in positions wherein radio frequencymagnetic lines of microwaves propagated by said microwave system passthrough and between said resonator elements.

11. A microwave apparatus comprising a plurality of substantiallyidentical disk-like elements constructed of a material characterized bya relatively high dielectric constant and high Q at microwavefrequencies:

said plurality of disk-like elements being positioned spaced from oneanother with their axes in alignment, and having the same resonantfrequency;

shield means forming an enclosure around said elements;

an opening in said shield means at an end of said shield means adjacenta first of said plurality of elements; and

means for applying microwave energy through said opening into saidenclosure to said element at a predetermined frequency to the one ofsaid elements at one end of said plurality of disk-like elements; and

there is included means for movably supporting said plurality ofdisk-like elements for enabling variation of the bandwidth of thefrequencies which can be propagated from said first to the last of saidplurality of elements inversely with the spacing between said pluralityof elements.

12. A microwave apparatus comprising:

a plurality of substantially identical disk-like elements constructed ofa material characterized by a relatively high dielectric constant and ahigh Q at microwave frequencies;

said plurality of disk-like elements being positioned spaced from oneanother with their axes in alignment and having the same resonantfrequency;

shield means forming an enclosure around said elements; an opening insaid shield means at the side of said shield means adjacent the centerof said plurality of disk-like elements; and

means for applying microwave energy through said opening into saidenclosure to said elements at a predetermined frequency substantiallysimultaneously to all of said disk-like elements; and

there is included means for movably supporting said plurality ofdisk-like elements for enabling variation of the frequency of resonanceof said structure.

13. A microwave apparatus as recited in claim 12 wherein each element isspaced from an adjacent element by a relatively thin dielectric materialof relatively low loss.

14. A microwave apparatus as recited in claim 12 wherein each element isenclosed in a cage of thin electrically conductive Wire loops disposedin meridional planes about said dielectric material.

References Cited UNITED STATES PATENTS 3,013,229 12/1961 De Grasse.2,948,870 8/ 1960 Clogston. 3,271,773 9/1966 Wheeler 333- HERMAN KARLSAALBACH, Primary Examiner PAUL L. GENSLER, Assistant Examiner Us. 01.X.R.

